The invention relates to a roller vane pump and in particular to a roller vane pump suited for pumping fluid in a continuously variable automatic transmission (CVT) for motor vehicles. Such a roller vane pump is known from the European patent 0.921.314 and is intended for pumping automatic transmission fluid in hydraulically controlled and/or operated continuously variable transmissions for motor vehicles. Particularly in a belt-and-pulley type CVT, a large flow of fluid at a high pressure may be required for control of the transmission. The known pump may be provided with several pump units, whereby a pump unit is a functional pump unit, i.e. having a suction section where fluid is drawn into the pump and a discharge section where fluid is discharged from the pump. Since the pump is usually driven by a main drive shaft of the vehicle, it is designed to be able to provide a desired pump yield, i.e. a desired flow of fluid, even at a lower most rotational speed of the drive shaft, e.g. idle speed. At the same time, the pump is designed to reliably withstand prolonged operation at an upper most rotational speed of the drive shaft.
The known pump is provided with a pump housing accommodating a substantially cylindrical carrier, which is rotatable about a central axis, and with a cam ring encompassing the carrier in the direction, whereby a clearance in the radial direction between the carrier and the cam ring varies along a circumference of the cam ring. The carrier is provided with a number of slots extending inwardly from the radially outer surface of the carrier, at least some of which slideably accommodate a roller element. The carrier is rotatable by means of a pump shaft extending co-axial with said central axis through the carrier. The pump shaft is supported in the housing on axial sides of the carrier, whereby the housing provides a bearing surface for the pump shaft. A small gap exists between the carrier and the pump housing as a result of an axial clearance introduced there between, which gap allows the carrier to rotate with respect to the housing. The gap further enables a lubrication flow from the discharge section of a pump unit to the bearing surface for lubrication thereof. It is noted that, as a consequence, the gap also enables a leakage flow from the high-pressure discharge section to the low-pressure suction section, which affects pump efficiency. Usually, said axial clearance will, therefore, be set as little as possible given a desired amount of lubrication.
The known pump has the disadvantage that the carrier may slightly tilt with respect to the pump housing under the influence of for instance mechanical shocks, changes in the rotational speed or changes in the fluid pressure at the discharge section. Particularly, when the pump shaft is relatively long a substantial movement of the carrier may occur. A rotation of the carrier causes said gap to vary along its circumference. At a location where said clearance is large, said leakage flow will also be large, whereby the -volumetric- pump efficiency is disadvantageously affected, whereas at a location where said clearance is small, possibly even non-existent, friction between the carrier and the housing is high, whereby the -mechanical- pump efficiency is again disadvantageously affected. At a location where said gap is non-existent, wear of the pump housing and of the carrier may also become a problem.
The above mentioned disadvantage of the known pump is particularly relevant when the pump housing is made of a light weight and/or soft material, such as aluminium, which is generally also a ductile material and/or when the carrier is rigidly fixed to the pump shaft. In such cases, said movement of the carrier may occur with relative ease, causing the width of said gap to change considerably along the circumference of the carrier. It is an object of the present invention to improve the pump efficiency of the known roller vane pump.
According to the invention this object is achieved with the roller vane pump, wherein there is provided in the pump housing a bearing bush having an essentially cylindrical central bore through which the pump shaft extends. In the pump according to the invention there is provided a separate bearing bush, which bearing bush accommodates the pump shaft in the pump housing and provides a bearing surface for the rotation of the said shaft. The provision of the bearing bush stiffens the construction of the pump shaft and thereby reduces said movement of the pump shaft.
In a further development of the invention, the bearing bush is made of material a less ductile than aluminium, such as copper. It is further preferred that the bearing bush tightly fits around said pump shaft in the radial direction. Both features have the advantage that the freedom of movement of the pump shaft is restricted. It may also be advantageous to provide a bearing bush on either axial end of the carrier. In this manner a stable configuration of the pump housing, the pump shaft and the carrier is achieved.
According to the invention it is advantageous, if the bearing bush is provided with a lubrication groove on a radially inner surface thereof, preferably having a substantially elongated shape with a long axis, for allowing a fluid to penetrate between the bearing bush and the pump shaft. To this end the lubrication groove may start at an axial end of the bearing bush closest to the carrier and continues with its long axis oriented in a direction having an axial component. The lubrication groove allows a flow of lubrication fluid in between the pump shaft and the bearing bush, even if the bearing bush fits relatively tightly around said shaft. The lubrication groove may span the entire axial length of the bearing bush. However, to prevent substantial fluid communication with the environment, it is preferred that the lubrication groove ends at some distance from the axial end of the bearing bush opposite said axial end of the bearing bush closest to the carrier. When the lubrication groove is oriented at an angle with respect to the axial direction, the lubrication fluid is distributed over at least a part of a circumference of the pump shaft. For optimum distribution of the lubrication fluid, the said angle is set such that the lubrication groove extends in the direction of rotation of the pump shaft.
As mentioned in the above, the pump is provided with one or more low pressure suction sections and one or more high pressure discharge sections, which sections are located alternately along the circumference of the cam ring. When said pressures are unevenly distributed along said circumference, a net-force acts on the carrier and on the pump shaft at a specific tangential location, which net-force urges the pump shaft in a generally radial direction. Thus, when the pump is provided with a single pump unit, or when the pump units have mutually different discharge pressures, a contact pressure between the pump shaft and the bearing bush is unevenly distributed in dependence on tangential position and varies between a highest level, at a tangential position substantially opposite a tangential position of the discharge section having the highest discharge pressure, and a lowest level, at a tangential position substantially corresponding to the tangential position of the discharge section having the highest discharge pressure. According to the invention, it is in such cases to be preferred that the lubrication groove is predominantly located in a region of tangential positions where the said contact pressure is relatively low, so that there is no need to disturb the contact between the pump shaft and the bearing bush at the location where the said contact pressure is the highest.
In a further elaboration of such embodiment, the lubrication groove starts at a tangential position of the discharge section where the prevailing pressure is at a maximum, whereby a tangential position corresponding to a central part of the said section is particularly suitable. If in such a case the long axis of the lubrication groove is oriented at an angle with respect to the axial direction, it is preferable that either one or both of a length of the lubrication groove and of said angle are chosen such that it extends in the tangential direction over an angle which approximately corresponds to xcfx80 minus xc2xdxcfx80 divided the number of pump units of the pump. This measure effects that the lubrication groove does not extend into the region of tangential positions where the said contact pressure is the highest.
In yet a further development of the pump according to the invention, the bearing bush is provided with a distribution groove on its inner surface having a substantially elongated shape with a long axis that is oriented substantially axially and that intersects the long axis of the lubrication groove, for further improving the distribution of the lubrication fluid. The distribution groove may extend over a substantial part of an axial dimension of the bearing bush. It is, however, to be preferred if there remains a distance of at least xc2xc of the said axial dimension between an axial end of the bearing bush and of the distribution groove so as to limit communication of lubrication fluid between the distribution groove and the environment.
The invention will now be explained in greater detail with reference to the non-restricting examples of embodiment shown in the drawings.